The present invention generally relates to ion beam systems, and in particular to a system and method for providing improved ion beam performance by mitigating power supply overload conditions.
Ion implantation is an important aspect of very large scale integration (VLSI) fabrication. Ion implantation is a process in which energetized, charged atoms or molecules are directly introduced into a semiconductor substrate. In VLSI fabrication, ion implantation is primarily employed to add dopant ions into the surface of silicon wafers. An objective of the ion implantation process is to introduce a desired atomic species into a target material. In order to meet this objective effectively, several aspects of the ion implantation process need to be controlled. First, the implant species should be implemented in the exact quantity specified. Additionally, the implanted species should be located at desired depths below the substrate surface and should be limited to only predetermined areas of the substrate. When required, it should be possible to electrically activate substantially all implanted impurities, and as much as possible, the silicon lattice structure should remain unchanged by the dopant incorporation process. Thus, ion implantation systems are needed that can accurately implant and monitor quantity and location of the implant species being implemented.
Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. This beam is directed to the surface of a workpiece. Generally, the energetic ions of the ion beam penetrate into the bulk of the workpiece and are embedded into the crystalline lattice of the material to form a region of desired conductivity. The ion implantation process is generally performed in a high vacuum, gas-tight process chamber which encases a wafer handling assembly and the ion source.
A typical ion beam path in prior art implantation systems includes an ion source, electrodes, an analyzing magnet arrangement, an optical resolving element, and a wafer processing system. The electrodes extract and accelerate ions generated in the ion source to produce a beam directed toward the analyzing magnet arrangement. The analyzing magnet arrangement sorts the ions in the ion beam according to their charge-to-mass ratio, and the wafer processing system adjusts the position of the workpiece relative to the ion beam path. The optical resolving element enables the system to focus ions having a selected charge-to-mass ratio, in conjunction with the analyzing magnet arrangement, so that ions are directed toward the workpiece.
Ion implantation systems generally provide high voltages to produce acceleration energies necessary to implant the ions into a substrate. Acceleration energies may range from 10-200 keV in many implantation systems to energies as high as several MeV in high-energy systems. Generally, such high voltages are applied via electrodes supplied by high voltage power supplies.
Often times, in conventional ion implantation systems, transient electric discharges may occur at electric field stress points, or discharges may be induced by particles or process deposits on electrodes and/or insulators. These discharges contribute to varying of electrode voltages, thereby affecting optics of the implantation system with potential loss of the ion beam. Consequently, the variation of electrode voltage may cause overloading and/or voltage collapse of power supply circuitry. Valuable ion beam implantation time may be lost during periods when power supplies are in an overload and/or voltage collapse condition. The duration of such periods is generally governed by the amount of time required for the power supplies to recover from the overload and/or voltage collapse.
In view of the above problems associated with ion beam loss in conventional ion implantation systems with regard to power supply overload from transient discharges, it would therefore be desirable to have a system and method which by mitigates such power supply overload and/or voltage collapse conditions.
The present invention provides a system and method for shortening ion beam loss times by hardening power supplies in ion implantation systems. Hardening is effectively accomplished by insulating the power supplies from electrode discharges associated therewith. As electrodes discharge during an implantation process, vacuum discharges associated therewith are quenched, and loss of energy stored in power supplies typically associated with such discharges is mitigated. Thus, the present invention significantly reduces the time required to recover electrode voltage and the associated ion beam.
More particularly, the present invention provides power supply hardening by insulating power supplies from transient discharges via a transient isolation system. The transient isolation system is operatively coupled between the power supplies and associated electrodes and/or other elements subject to discharge. The transient isolation system of the present invention serves to temporarily insulate power supplies from electrode circuits during transient discharges associated with the electrodes. Without transient isolation, discharges generally cause electrode power supply to be discharged such that electrode voltages drop to a low value, thereby causing the power supplies to operate in an overload (e.g, over-current, under-voltage) condition while feeding the discharge. It has been observed that ion beam losses may last for several tens of milliseconds while power supplies recover from an overload condition. By isolating the power supplies, the present invention enables power supply current and voltage to remain at acceptable levels even during a discharge situation. Since the power supplies are isolated during transient discharge conditions, discharges are substantially only fed by stray capacitance associated with the electrodes during a few microseconds, and thus substantially quenched by lack of sufficient power from the isolated power supplies.
According to one aspect of the present invention, an ion implantation system is provided. The system includes at least one power supply for providing voltage to at least one electrode and a switching system operatively coupled between the at least one power supply and the at least one electrode. The switching system decouples the at least one power supply and the at least one electrode at a predetermined threshold to mitigate overload of the at least one power supply.
According to another aspect of the present invention, an ion implantation system is provided. The system includes an ion source for providing ions to form an ion beam, and at least one power supply for providing voltage to at least one electrode for directing the ion beam to a workpiece. The system includes a transient isolation system for providing a substantially uninterrupted ion beam to the workpiece. The transient isolation system insulates the at least one power supply from discharges by the at least one electrode.
According to yet another aspect of the present invention, an ion implantation system is provided. The system includes a voltage source for providing voltage to at least one electrode, and a means for decoupling the voltage source and the at least one electrode at a predetermined threshold to mitigate overload of the voltage source.
According to still yet another aspect of the present invention, a methodology for power supply hardening in an ion implantation system is provided. The methodology includes the steps of: monitoring current from at least one power supply which powers at least one electrode; determining if the current is below a predetermined threshold; and insulating the at least one power supply from the at least one electrode if the current is above the predetermined threshold.
According to still yet another aspect of the present invention, a methodology for power supply hardening in an ion implantation system is provided. The methodology includes the steps of: monitoring voltage from at least one power supply which powers at least one electrode; determining if the voltage is below a predetermined threshold; and insulating the at least one power supply from the at least one electrode if the voltage is above the predetermined threshold.